Recently, great attention has been paid to ionic liquid-incorporated polymer electrolyte membranes (PEMs) for a variety of applications such as electroactive polymer actuators, lithium batteries, high-temperature fuel cells, etc. Thorough research thereto is ongoing as in the citation list described later.
Ionic liquids are organic salts, and have various physicochemical properties such as a low melting temperature, negligible vapor pressure, high thermal stability, electrochemical stability, etc. Because of such properties, ionic liquids are regarded as an alternative to volatile solvents that are typically used for high-temperature fuel cells.
Also, ionic liquids are advantageous because the degree of ion dissociation, local concentration of ions, and glass transition temperature of polymer electrolyte membranes may be changed when polymer electrolyte membranes are impregnated therewith, thus controlling the thermodynamic and electrochemical properties of polymer electrolyte membranes.
With the goal of enhancing performance of polymer electrolyte membranes, 1,3-alkylimidazole [CnIm, n=2˜11]-based ionic liquids have been utilized to date. Examples of the polymers for use in electrolyte membranes may include poly (vinylidene fluoride) (PVDF), poly (ethylene oxide) (PEO), poly(methyl methacrylate) (PMMA), poly(vinyl pyridine) (PVP), and poly (styrene sulfonate) (PSS).
Furthermore, methods developed to increase conductivity of the polymer membranes include increasing the local concentration of ions present in the conducting domain of the polymer, increasing the length of the alkyl chain in [CnIm] cations, altering the anion of ionic liquid, and enhancing the segregation between the ion conducting domain and the non-conducting domain in the polymer electrolyte membrane.
For example, Korean Patent No. 1119532, filed by the present inventors, discloses a method of increasing conductivity of a block copolymer including a sulfonated block using an ionic liquid composed of an anion containing a sulfur element and an imidazole-type cation.
However, the polymer electrolyte membranes impregnated with alkylimidazole [CnIm]-based ionic liquids have insufficient protic sites where protons may be efficiently transported while being donated to or received from the [CnIm]-based ionic liquids, making it difficult to enhance performance of the membranes.